Electromagnetic induction contactless power supply systems have become practical.
An electromagnetic induction contactless power supply system includes a contactless power supply device having a primary coil and a power reception device having a secondary coil. When an electric appliance including the power reception device is set on a setting surface of the contactless power supply device, the contactless power supply device excites the primary coil. Then, electromagnetic induction excites and supplies power to the secondary coil of the power reception device, which is arranged in an electric appliance. The secondary power generated in the secondary coil is converted to direct current (DC) power in the power reception device. The DC power is then supplied to a load of the electric appliance as driving power.
If a metal foreign object is present between the contactless power supply device and the electric appliance (power reception device), the metal foreign object may be inductively heated when power is being supplied. Accordingly, the contactless power supply device may include a metal detector that detects metal foreign objects. When the metal detector detects a metal foreign object, the contactless power supply device stops supplying power.
In one example, the metal detector in the contactless power supply device excites the primary coil of the contactless power supply device at a predetermined frequency, which differs from the frequency used when power is supplied. It is known that the presence of a metal foreign object changes the inductance of the primary coil, which is excited at the predetermined frequency. When detecting the presence of a metal foreign object based on the change in the inductance of the primary coil, the metal detector stops the supply of power (Japanese Laid-Open Patent Publication No. 2000-295796)